The US estimates a much higher drop in opium than the UN
Opium production in Afghanistan will fall by almost a third this year, according to a US government report.
The White House Office of National Drug Control Policy report estimates opium production will plunge by 31% to 5,500 tonnes compared to 8,000 in 2007.
The United Nations, however, estimates the drop in opium production will only be 6% this year.
Ninety percent of the world's opium - which is used to make heroin - comes from Afghanistan.
"We are very pleased to announce today the US government estimate of poppy cultivation and opium production in Afghanistan showed substantial declines," said John Walters, director of the White House Office of National Drug Control Policy.
Mr Walters said poppy cultivation will fall by 22%, similar to estimates given by the UN Office on Drugs and Crime in August.
But while the UN believes there has been a rise in yields in fields still under cultivation, the Bush administration says yields have fallen.
Mr Walters suggested that discrepancies with the UN prediction were due to different methodologies in compiling the data.
He said 18 of Afghanistan's 34 provinces were now no longer cultivating the heroin precursor, compared with 15 previously.
The main reason for the decrease in production is drought, along with anti-drug campaigns.
"It gives us a clear indication that we can do this, we just need to sustain it," Mr Walters said, referring to the successful anti-drug campaigns in the north and east of the country.
But the southern Helmand region, where Taleban rebels are active, accounts for nearly 66% of all the opium production.
Trade in the drug is helping to fund the militants and Mr Walters said the militants and the cultivation of opium, alongside corruption, had to be tackled together.